It is generally known that the permanent magnet motor has the following advantages: it is comparatively simple in structure, reliable, smaller in size, lower operation loss, high operation efficiency, and can be designed with various shapes and sizes, by that permanent magnet motors can be adapted for various applications in many fields, including aerospace industry, defense industry, agriculture, and so on. However, the permanent magnet motor is still short in the performance relating to trapezoidal-shaped back electromotive force (EMF) and cogging torque. Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a permanent magnet motor, and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator. Cogging torque is an undesirable component for the operation of such a motor. It is generally prominent at lower speeds, with the symptom of vibration and noise during operation. Cogging torque is especially obvious in the operation of a outer-rotor permanent magnet motor, since the rotor magnets of a outer-rotor permanent magnet brushless motor are attached to an inner surface of its rotor yoke in a surface-mount manner, resulting the waveform of the outer-rotor permanent magnet brushless motor to be a trapezoidal wave.
Conventionally, most prior arts and disclosed papers for reducing cogging torque and modifying back-EMF are performed based upon the arrangement of rotor magnets in an inner-rotor permanent magnet brushless motor. Nevertheless, the arc-shaped rotor magnet in an outer-rotor permanent magnet brushless motor will cause high manufacturing cost for modifying such rotor magnet and difficulty in tolerance control as well.